There is an ongoing desire within the law enforcement communities to have a way of recognizing or tracing explosives that have been used in a crime. It will be appreciated that a useful taggant will need to satisfy a number of challenging performance requirements. First, the taggant material must be easily and cheaply dispersed throughout a batch of explosive material using traditional formulating and blending equipment. At the same time, explosives, by their very nature, must be formulated carefully and the industry is understandably reluctant to add materials that might make them unstable or unpredictable. In particular, taggants made from small bits of plastic might be a source of static electricity that could lead to catastrophic detonation during processing, mixing, or loading cartridges. Users will also balk if the performance of the explosive is degraded or if significant costs are added. For the taggant particles to be useful, they must be easily recovered at a crime scene. This means they must first survive and then be detectable (found and collected) at the scene with simple equipment. They must also retain enough of their original distinctive properties to meet the standards of legal evidence. Among other things, this means that the taggant particles must be easily distinguished from other natural or man-made particles such as minerals, cement, fly ash, smelting residues, and the like.
Several methods for tagging sensitive materials such as explosives have been developed. These utilize colored or magnetic particles that can be recovered in the field and then “read” by recognizing some distinguishing characteristic such as the pattern of colored bands or a distinctive spectral response.
Powdered phosphor materials potentially satisfy several of the aforementioned requirements for use as taggants. They are bright (i.e. only a small amount is needed in order to be detected), each material exhibits its own unique optical characteristics that can be easily detected, and they are compatible with common bullet and propellant materials. Some of the best candidate materials are ceramics, and are thus capable of enduring harsh environments with no impact on their optical functionality.
In U.S. Pat. No. 3,772,099, dated Nov. 13, 1973 a fluorescent tagging means for pre-detonation or post-detonation identification of explosives is described. A combination of “spotting” phosphor, which is excitable by ultraviolet radiation to provide a band-type emission and “coding” phosphors, which emit a line-type emission, were added to explosives. All of the phosphors have very small particle sizes and are incorporated into phosphor grains held together by inorganic cement or a binder, and the resulting conglomerate grains are dispersed in the explosive. After an explosion, the phosphor grains can be readily located by the ultraviolet response of the spotting phosphor, and information disclosed by the presence (or absence) of the coding phosphor can then be decoded. The spotting phosphor will also normally provide some additional indicia of information. Further refinements of this tagging system are disclosed in U.S. Pat. No. 3,967,990, dated Jul. 6, 1976.
To inhibit any tendency for the phosphor conglomerate particles to sensitize the explosives, the individual particles were encapsulated in an organic plastic material such as polyethylene and a method for so encapsulating these particles are disclosed in U.S. Pat. No. 3,961,106, dated Jun. 1, 1976. These encapsulated particles can incorporate an anti-static coating, such as amorphous carbon, in order to prevent electrostatic charging thereof, and such a coating is described in U.S. Pat. No. 3,993,838, dated Nov. 23, 1976.
U.S. Pat. No. 3,897,284, dated Jul. 29, 1975, discloses microparticles for tagging of explosives, which particles incorporate a substantial proportion of magnetite that enables the particles to be located by means of magnetic pickup. Ferrite has also been used. More recently, modified tagging particles with strips of color coding material having a layer of magnetite affixed to one side and layers of fluorescent material affixed to both exterior sides, has been developed. In this system, the taggant can be located by visual detection of the luminescent response, or magnetic pickup, or both. Both the ferrite and the magnetite materials are, however, dark colored and absorptive of the radiation which excites the luminescent material, thereby making the particles somewhat difficult to locate after an explosion. Further developments produced similar particles that take advantage of the magnetic properties without diminishing the luminescent response of the materials (U.S. Pat. No. 4,131,064).
Yet, another approach is the development of particles coded with ordered sequences of distinguishable colored segments (U.S. Pat. No. 4,053,433). Other patents employ radioactive isotopes or other hazardous materials as taggants.